Air Seeding machines include a metering device which regulates that amount of seed and fertilizer that is dispensed into an air stream. The air stream conveys the seed and/or fertilizer to a secondary tower which divides the flow of materials into individual row air streams for delivery to furrows made in soil by a furrow opener. The current technology metering devices meter the seed or granular fertilizer on a volumetric basis. In order to achieve an acceptable degree of metering accuracy, the metering device must be calibrated in accordance with the density of the material being metered. The calibration procedure, which typically includes a manual weighing step, can be time consuming and, depending upon the skill level of the operator, can be inaccurate and result in reduced productivity. When more than one material is being conveyed, only one of the materials can be metered at a time during the calibration procedure and presents added difficulties when attempting to provide a calibration system that can operate on-the-go.